I've got a 4 speed that goes to that engine, Ben. I'm way too far from you, though.
If you check on
PeachParts - Mercedes Parts & Tech Info (I have the same UN there), you'll likely find someone close that not only has the manual transmission you need, or can tell you a slough of other transmissions that can be used (such as the gearbox from early BMW 5 and 7 series) for your project, but also someone that might be interested in buying parts from your MB donor car, setting you ahead a bit further.
If you're going to be in this area (NEPA) ever, shoot me a PM and we can talk about the 4 speed that goes to that engine. I have the same 62HP OM616 engine sitting in my barn right now, waiting for Cara to come off the road. (Potentially.)
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